Certain carbides, nitrides, borides, oxides, and suicides exhibit enhanced mechanical properties, including damage tolerance and wear resistance. As a result, these materials have found use in dynamic environments where the materials are subject to harsh conditions, such as increased wear, thermal shock, elevated temperatures and the like. For example, many of the carbides, nitrides, borides, oxides, and silicides of the elements from Groups IVb, Vb, and VIb of the periodic table, as well as carbides, nitrides, borides, oxides, and silicides of boron, aluminum, and silicon have been used in industrial and other applications where such conditions are likely to be encountered. Generally, structures formed of these materials exhibit improved strength and hardness at ambient and elevated temperatures, improved toughness and wear resistance, high melting points, thermal shock resistance, and oxidation resistance.
These materials have found use in the fabrication of structures that may be subject to impact damage by foreign objects, which is commonly referred to as foreign object damage (FOD), such as in turbomachinery and turbine engine applications. These materials also may be used for the fabrication of tools, inserts and other implements that may be subject to wear and/or impact damage, such as in mining, construction, machining of metals and composite materials and similar industrial applications. For example, because of their abrasive, impact and wear resistance properties, materials formed of carbides, nitrides, borides, oxides, and silicides have been used in mining applications where, for example, extremely severe wear conditions and impact loadings may be encountered by the drill bits during rock crushing and removal. Additionally, because of their ability to withstand high temperatures, these materials also have been used in machining applications where very high localized temperatures may be encountered adjacent the cutting edge of the tool.
The usefulness of such materials, however, has been limited by a lack of wear resistance, damage tolerance and fracture toughness exhibited by the materials. As a result, structures fabricated from these materials tend to wear and/or fracture more quickly than is desired. The need for frequent replacement of parts is costly and results in down time that is both time consuming and costly.
Metallic and ceramic cutting tools, for example, are subject to harsh conditions during use that can result in unpredicted failure of the tool. The challenges in predicting when the various cutting tools in a machine will fail and their useful life, among other problems, can become very costly in terms of equipment downtime and replacement costs. In order to overcome the challenges associated with metallic and ceramic cutting tools, a separate coating material may be applied to the outer surface of the cutting tool to provide additional wear, crater and heat resistance.
One process for coating cutting tools and other wear surfaces is chemical vapor deposition, or “CVD,” where a coating is applied to a substrate in a hot-wall reactor. The coating material typically is produced by reactive deposition from a gaseous phase at elevated temperatures. For example, one coating material, TiC, is produced when TiCl4 vapor is mixed with CH4 at approximately 1000° C. and converted to fine-grained TiC crystals. Coating thicknesses produced by CVD range between about 5 to about 20 μm. Multiple layers of different coating materials often are required to coat a substrate in order to obtain the desired functional properties. A typical CVD cycle can last 24 hours or more from load to unload, making the process very time consuming. Additionally, CVD processes require ancillary equipment that is costly and involves potentially environmentally hazardous gases and chemicals.
Another process for coating cutting tools and other wear surfaces is physical vapor deposition, or “PVD,” where a vaporized or ionized compound is deposited within a vacuum chamber. Common PVD methods include vacuum vapor deposition by arc melting, crucible melting, cathodic sputtering and reactive sputtering. Arc melting provides the fastest deposition rate, while cathodic and reactive sputtering are relatively slow processes in which about 1 μm of coating is deposited per hour. Crucible melting can only be used with pure metals and compounds that do not dissociate on evaporation. Coating thicknesses produced by PVD range between about 2 to about 10 μm. PVD processes provide the advantage of lower processing temperatures (about 400° C.) as compared to CVD processes. Similar to CVD, however, PVD processes require ancillary equipment that is costly and involves potentially environmentally hazardous gases and chemicals.
In addition to the equipment and maintenance requirements associated with CVD and PVD processes, other shortcomings are associated with these processes. As with any discrete coating material applied onto another, different material, concerns relating to the thickness of the coating (including the desire to prolong the life of the coating by increasing its thickness while not impairing the physical structure of the substrate), the compatibility of the coating with the substrate, the ability of the coating to provide the desired material properties, and the strength of the bond between the coating and the substrate also exist.
There remains a need for materials exhibiting improved hardness, strength, wear resistance and fracture toughness, as compared to presently known materials, for use in dynamic environments to mitigate impact damage to, and/or increase the wear resistance of, structures comprising such materials. There also remains a need for materials exhibiting improved wear resistance and coating durability as compared to materials coated using conventional processes, including CVD and PVD.